I have never built a spud gun before, so I am starting slow and simple. My plan is to build a pneumatic air cannon. I think I have the design down, but i would like to know how to install the valve in which the air is pumped into. i plan on using a bike pump, and a copper valve. The cannon will be a single rod, divided by a ball valve. The compression chamber will be 4" while the barrel will be 3". Most likely, the whole cannon will be about 4 feet long or so.

It should look something like this:

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that is only the rear where the compressed air will be held. The samll point sticking out of the top is where id like the valve to be.

First of all welcome to the sport my friend.
To answer your question it would be best to get reducers, but if you can’t (if you like me your going cheep) screw it in the area were the end cap is attached to the presser chamber where it is the thickest. Good luck, have fun.

How to install the valve-
-The to valves it sounds like youd be willing to use is this 'begineer' if you will cannon is a ball valve and a sprinkler valve. [I tell you right now PVC ball valves suck anus, there very hard to turn] Both the valves have female threads to thread into. So off one end of the barrel, and one end of the chamber, you will need male adapter fittings that will fit the thread on you valve. Id suggest you use a modded sprinkler, and knock down the chamber diameter. Make sure to use teflon tape on the threaded connnections.

How to install the air valve-
-Schrader valves, and air compressor quick disconnects are the common ways of adding air into your chamber. Most commonly the QC's have 1/4" thread, and Schraders with 1/8" and 1/4" NPT threads. This device should be tapped into a area in which a pipe and fitting overlap.

About the 'reducers'-
Reducers have shorter socket depths, which can be a concern when using prolonged pressures. One should use <a href="http://www.watertanks.com/images/sch80reduc.jpg">bushings</a>, instead of <a href="http://media.doitbest.com/products/408948.gif">reducers</a>.

ok thanks, well i launched it yesterday and today. ill post pictures as soon as i take them, but it got a golf ball 80 yards. even though thats not to far, you have to remember that the chamber is the same size as the barrel, and that its leeking. im working on stopping that now, i have the chamber screw into the cannon itself, that way i can change chambers. i tryed to use none permenant pipe sealent, but air excapes. i was wondering if there is another sealent i could use instead of gluing the chamber shut.
thanks-